The disclosed subject matter relates generally to telecommunications and, more particularly, to a shared protection scheme for multiple variable batteries.
In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A device at the subscriber station, such as a cable modem, typically implements one or more subscriber line interface circuits (SLICs) for interfacing with the subscriber line.
The SLIC provides an interface between a low voltage signal path in the subscriber premise and a high-voltage telephone subscriber line. The SLIC provides functions such as off hook detection, ringing signal generation, and battery feed to the subscriber line. The subscriber line consists of a telephone transmission line, including two conductors referred to as A and B or tip and ring, and the subscriber telephone equipment coupled across the tip and ring conductors (i.e., the load). The subscriber line and the subscriber telephone equipment are also referred to as a subscriber loop.
The subscriber premises device generates battery voltages for the operation of the SLICs. The battery voltage is a DC voltage supplied to the SLIC to power the SLIC and the subscriber line. The SLIC supplies a DC current at the battery voltage to the subscriber line. Superimposed on the DC current are AC signals of audio frequency by which information is conveyed between the subscriber and the central office or a local premise device, such as a cable modem.
It is common to use multiple battery voltages in the SLIC, such as a positive battery, VBP, and a negative battery, VBH. To save power and reduce power dissipation on the SLIC for short loop applications, a third supply is often used for DC feed along with VBH. The third supply is typically lower in amplitude than VBH, and is often referred to as VBL.
In some applications, one negative battery (e.g., VBL1) is used to power a first channel, and a second negative battery (e.g., VBL2) is used to power a second channel. For example, cable modems may be implemented using this technique, where a dual-channel modem may have two SLICs, or separate cable modems may operate on the same subscriber line. Each battery provided to the SLIC (e.g., by a device such as a cable modem) will dynamically vary its amplitude or value according to the SLIC operation condition.
To protect the SLICs from voltage excursions, such as lightning surges or power cross events, a protection scheme is employed to connect the tip or ring lines experiencing the event to ground. Because of the different voltages used, one protector is typically employed to track the VBL1 supply and another protector is employed to track the VBL2 supply. Each protector includes protection circuitry and a gate capacitor to handle fast transients. This multiple-protector protection scheme is costly, since each channel requires its own protector and a relatively expensive gate capacitor.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.